Packet ring networks include an RPR (Resilient Packet Ring) standardized according to IEEE 802.17. The RPR is an MAC layer protocol for providing access to ring-like transmission mediums, and is capable of realizing quick failure recovery for devices in the category of carriers, effective network band utilization, and shortest route transfer.
FIG. 1 is a diagram illustrative of an RPR network configuration. As shown in FIG. 1, an RPR network includes a packet ring having two ringlets 151, 152 for transferring packets in mutually opposite directions. The packet ring interconnects a plurality of nodes in a ring fashion.
In the example shown in FIG. 1, four nodes 153a, 153b, 153c, 153d are connected to the packet ring. Each of the nodes on the packet ring is assigned an RPR MAC address. When the network is established, the nodes exchange control packets, and each of the nodes collects information about the number of hops and acquires topological information of the network.
A user terminal may be connected to each of the nodes on the packet ring. In the example shown in FIG. 1, user terminal 154a is connected to node 153a and user terminal 154b is connected to node 153b. 
An RPR data packet standardized according to IEEE 802.17 will be described below.
FIG. 2 is a diagram illustrative of an RPR format. When a user terminal sends a packet to a node, it sends user data packet 211. User data packet 211 includes MAC address (MAC DA) 212 of a user terminal as the destination of the user data packet, MAC address (MAC SA) 213 of the user terminal as the source of the user data packet, transmission data 214, and FCS (Frame Check Sequence) 215.
When the node receives the user data packet from the user terminal, the node encapsulates the user data packet to generate RPR data packet 221, which is sent and received between the nodes. RPR data packet 221 includes encapsulated user data packet 211 that is stored as data 226.
RPR data packet 221 includes MAC address (RPR MAC SA) 225 of a destination node, MAC address (RPR MAC DA) 224 of the source node, Base Control field 223, TTL (Time To Live) field 222, and FCS 227.
Base Control field 223 includes information for specifying a ringlet to be used for transfer and identification information for identifying the type of a packet such as a control packet or the like. Details of the RPR data packet format are described in Non-patent Document 1.
The operation of each node on the packet ring to send, receive, and transfer an RPR data packet will be described below.
First, the operation with respect to a unicast data packet will be described below. Each node receives an RPR data packet transferred on the packet ring. If the RPR MAC DA of the RPR data packet is the same as the RPR MAC address of its own, each node deletes the RPR data packet from the packet ring.
If the RPR MAC DA of the received RPR data packet is different from the RPR MAC address of its own, each node decrements the TTL value (the value set in TTL field 222), and sends the RPR data packet again to the same ringlet as the ringlet from which the RPR data packet has been received. If the source node receives a unicast data packet sent thereby, the source node deletes the unicast data packet from the packet ring. When the TTL value becomes nil, each node deletes the RPR data packet from the packet ring.
The operation with respect to a broadcast data packet will be described below. After decrementing the TTL value of a received broadcast data packet, each node transfers the broadcast data packet to a next node. If the source node which has sent the broadcast data packet receives the broadcast data packet sent thereby, the source node deletes the broadcast data packet from the packet ring. When the TTL value becomes nil, each node deletes the RPR packet from the packet ring.
An RPR control packet (hereinafter referred to as “control packet”) standardized according to IEEE 802.17 will be described below.
In order to make all nodes belonging to an RPR network able to perform autonomous functions including a topology discovery function, a protection function, and OAM (Operation, Administration, and Maintenance) function, each RPR node sends and receives a control packet via a data path.
FIG. 3 is a diagram illustrative of a configurational example of a conventional node in a packet ring network. In the example shown in FIG. 3, ringlet-0 data path 10 and ringlet-1 data path 20 send a generated control packet to respective ringlets, and receive control packets from the ringlets. The control packets are individually defined with respect to the above functions according to IEEE 802.17. Packets are transferred in ringlet-0 data path 10 and ringlet-1 data path 20 in the same manner as the RPR data packet is transferred as described above.
The operation of the RPR network shown in FIG. 1 for user terminal 154a connected to node 153a to send data to user terminal 154b connected to node 153b will be described below.
Each node learns MAC SA 213 (see FIG. 2) of the source user terminal, which is encapsulated in the received RPR data packet, and source RPR MAC SA 225 (see FIG. 2) in association each other, and holds a database of RPR MAC addresses accessible using the MAC addresses of the user terminals as a retrieval key. The database of RPR MAC addresses accessible using the MAC addresses of the user terminals as a retrieval key will be referred to as FDB (Filtering DataBase).
When user terminal 154a sends data (user data packet) to the packet ring, node 153a receives the user data packet. Using MAC DA 212 (see FIG. 2) of the received user data packet as a retrieval key, node 153 searches the FDB, and uses the result as RPR MAC DA 224 (the MAC address of the destination node, see FIG. 2).
Node 153a uses its own MAC address as RPR MAC SA 225 (the MAC address of the source node, see FIG. 2). Node 153a encapsulates the user data packet received from user terminal 154a. Furthermore, node 153a searches a topology database, selects a ringlet for providing the shortest route from the source node to the destination node, sets a TTL value, and sends an RPR data packet to the packet ring.
If the association of the MAC address of the user terminal as the destination and the RPR MAC address corresponding to the MAC address has not been learned as a result of the searching of the FDB, then node 153a performs flooding. The RPR MAC DA of the RPR data packet sent by the flooding is set to a broadcast address, so that the RPR data packet is received by all the nodes on the packet ring.
As a result of the flooding, the user data packet sent by user terminal 154a is received by destination user terminal 154b. Normally, user terminal 154b replies to user terminal 154a in a higher-level layer. At the time of replying, user terminal 154b serves as the source of the user data packet, and user terminal 154a as the destination. Node 153b serves as the source of the RPR packet.
When user terminal 154b replies, node 153a learns the association between the MAC address of user terminal 154b and the RPR MAC address of node 153b. Therefore, when user terminal 154a sends a user data packet again to user terminal 154b, node 153a searches for the RPR MAC address of node 153b using MAC DA 212 included in the user data packet as a key, and can perform a unicast transfer using the search result as RPR MACC 224.
Processes for flooding a broadcast packet to the packet ring include a process in which the source node sends the broadcast packet to an arbitrary one of the ringlets and a process in which the source node sends the broadcast packet to both ringlets and transfers the broadcast packet to a reaching point which has been established in advance on the packet ring for the prevention of a multiple transfer (bidirectional flooding).
The reaching point which has been established in advance on the packet ring for the prevention of a multiple transfer is referred to as a cleave point. According to the bidirectional flooding, depending on whether the number of nodes on the packet ring is an even number or an odd number, it is necessary to change the process of calculating the TTL value so that a packet will be transferred to all the nodes and will not arrive doubly at the nodes. The TTL value calculating process will not be described below as it has little bearing on the present invention.
A TP frame will be described below. The TP frame is a fixed-length frame and serves as a control frame for indicating information representative of the state of span protection and edge of a node and a sequence number to all the nodes other than its own node. The number of nodes making up the packet ring network is set in the TTL value, and the TP frame is broadcast to both ringlet-0 and ringlet-1. Each node collects the information of TP frames received from all the nodes other than itself, and constructs a topology database.
The protective operation of an RPR at the time of a link failure will be described below with reference to FIGS. 4A through 4C. FIGS. 4A through 4C are diagrams showing an example of a packet ring network in the event of a failure occurring in a link.
According to IEEE 802.17, a steering mode and a lap mode are defined for the protective operation at the time of a failure. The steering mode is defined as an essential function, and the lap mode as a selective function. The steering mode and the lap mode are referred to in Patent Document 1.
FIG. 4A is a diagram illustrative of an ordinary network operation. In FIG. 4A, a packet is transferred on ringlet 301 from node 303a to node 303b. 
FIG. 4B is a diagram illustrative of a steering mode of operation. As shown in FIG. 4B, in the event of the occurrence of failure point 304, all the nodes on the packet ring obtain the positional information of failure point 304. Specifically, nodes 303c, 303d that are connected to the link containing failure point 304 indicate the positional information of failure point 304 to all the nodes. As a result, each node recognizes the position of failure point 304.
For sending a unicast packet, the source node selects a ringlet which does not include failure point 304 between itself and a destination node to which an RPR packet is to be sent, and sends the unicast packet.
For example, if node 303a is to send a unicast packet to node 303b, node 303a recognizes the position of failure point 304 and, based on the recognized position, changes the ringlet for sending the unicast packet from ringlet 301 to ringlet 302, and transfers the packet to node 303b. For sending a broadcast packet, node 303a selects both ringlets 301, 302 and sends the broadcast packet to ringlets 301, 302. As a result, the broadcast packet is sent to each node on the packet ring.
FIG. 4C is a diagram illustrative of a lap mode of operation. In the lap mode, the source node selects the same ringlet as with the normal operation and sends an RPR packet. For example, if node 303a is to send an RPR packet to node 303b, node 303a selects ringlet 301 as with the normal operation (see FIG. 4A) and sends an RPR packet.
Node 303c is connected to the link containing failure point 304 and has detected the failure. When node 303c receives the RPR packet, node 303c selects ringlet 302 which is different from the ringlet through which the packet has been sent, and transfers the RPR packet through ringlet 302.
Specifically, node 303c transfers the RPR packet to an area which is free of failure point 304. The packet is transferred through ringlet 302 to node 303d that is connected to the link containing failure point 304 and has detected the failure. Node 303d also selects the ringlet which is different from the ringlet through which the packet has been sent, and transfers the RPR packet through the ringlet. As a result, node 303b receives the RPR packet.
FIGS. 5A through 5C are diagrams showing an example of a packet ring network in the event of a failure occurring in a node. As shown in FIGS. 5A through 5C, the protective operation of the RPR in the event of a failure occurring in a node is the same as the operation in the event of a failure occurring in a link (see FIGS. 4A through 4C).
As described above, the protective operation of the RPR according to IEEE 802.17 makes it possible to realize quick failure recovery from a span failure and quick failure recovery of inter-node communications from a node failure except for a node that is suffering the node failure. However, it fails to define a redundant configuration for RPR nodes, and when a failure occurs on a node, the node loses its connectivity to a client apparatus to which the node has been connected, and is unable to communicate with the client apparatus.
There is no definition either about failure recovery from a link failure with respect to a link by which a client is connected to an RPR node. When such a link failure occurs, the node loses its connectivity to the client apparatus, and is unable to communicate with the client apparatus.
Furthermore, in either the steering protection mode or the lap protection mode, the ring transfer band is narrower than in the normal operation.
Generally, network devices in the category of carriers need to have a redundant configuration for each card or each device, and are required to recover quickly from a failure occurring in the card, the device, and their connection.
According to IEEE 802.17, it is assumed that the node shown in FIG. 3 is connected as a single node to a client apparatus. There is no definition about the connectivity between an RPR and a client apparatus in the event of a failure. If the node shown in FIG. 3 is installed as a hardware unit such as a single integrated circuit, card, or device, then in the event of a failure of the hardware unit or the connection to the hardware unit, a disturbance such as a packet transfer route change, a utilizable band reduction, or the like affects other packet ring nodes, and the connectivity of the node to its client apparatus is lost.
Patent Document 2, Patent Document 3, and Patent Document 4 disclose a node configuration including an active node which is operable in a normal state and a backup node which is not operable in the normal state. According to the node configuration, the active node switches to the backup node in the event of a failure of the active node.
Patent Document 1: JP-A No. 2004-242194 (paragraph 0004, paragraph 0012, FIG. 1)
Patent Document 2: JP-A No. 4-100446 (p. 4-5, FIG. 1)
Patent Document 3: JP-A No. 2005-130049 (p. 10-11, FIG. 1)
Patent Document 4: JP-A No. 2005-27368 (p. 6-8, FIG. 1)
Non-patent Document 1: “IEEE Standards 802. 17, Part 17: Resilient packet ring (RPR) access method & physical layer specifications”, “5. Architecture Overview”, “6.6.1 MAC Control sublayer”, “9. Frame formats, IEEE (Institute of Electrical and Electronics Engineers, Inc), 2004, p. 27-54, p. 68-69, p. 221-223